Takashi Kiyota

On 28th September 2018, an earthquake of Mw 7.5 struck Donggala Regency, Central Sulawesi, Indonesia, resulting in a catastrophic disaster and a large numbers of casualties. In this report, the results of the preliminary field investigations carried out by the authors’ team are summarized. The earthquake caused four major long-distance flow-slides on very gentle slopes of approximately 1–5% in and around Palu City: Balaroa, Petobo, Jono Oge and Sibalaya. The maximum flow distance at each site wa

This study investigates the effects of wetting and drying cycles, known as slaking, on the strength-displacement characteristics of crushed mudstone. A series of direct shear tests was conducted by simulating cyclic wetting and drying under different stress conditions using a modified direct shear apparatus. The effects of the stress ratio, the density of specimen, the initial water content before wetting, the slaking index, and the number of wetting and drying cycles on the test results was inv

Some previous studies have shown a good correlation between the shear wave velocity, Vs, and the cyclic resistance ratio, CRR. Recently, however, a Vs-based liquefaction assessment method has become an alternative and supplementary method to the conventional NSPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric

This study investigates the effects of quality of in-situ sandy samples, which were taken by a conventional tube sampling (TS) method, on liquefaction assessment. The sandy samples were retrieved from a loose fill layer and medium-dense Holocene deposit in young reclaimed land in Urayasu City where significant liquefaction occurred during the 2011 off the Pacific Coast of Tohoku Earthquake. First, a series of small strain measurements and undrained cyclic triaxial tests was conducted on the TS a

Drained and undrained creep characteristics of saturated loose fine sand were compared and interpreted based on the yielding and viscous properties under general stress conditions. The viscous properties were evaluated by performing specially designed triaxial compression (TC) and extension (TE) tests. To quantify the yield loci for shear and compression yielding mechanisms on the p'-q plane, various loading histories consisting of pre-shearing and pre-isotropic compression were applied. Differe

これまで補強土構造物にアンカー力を生かした引張り補強材としてジオセルが採用された例はほとんどなく、また独特の構造的特徴があるジオセルが盛土内の引張り補強材としてどの程度効果的に機能するのか検討した研究もほとんどない。本研究では、引張り補強材としてのジオセルの機能を検討することを目的とした。ジオセルの高さと敷設長を変えた引抜き試験により、ジオセルの引き抜け抵抗はジオセルの高さよりも敷設長に依存することを確認した。また、ジオセルは引張り力を加えた先端から深部に向けて進行的に変形したが、その傾向は敷設長により異なった。これら一連の実験から、敷設されたジオセル全体の平均的引張り剛性が高くなり進行的変形が小さくなれば効果的に引抜け抵抗を増加できることが分かった。そのようにすれば、ジオセルは鉛直方向に強度・剛性を持ち大粒径の盛土材に適用できる引張り補強材として利用できる可能性がある。

地盤の液状化強度比の評価にあたっては、標準貫入試験に基づく簡易評価法と不撹乱試料を用いた非排水繰返し載荷試験（液状化試験）結果を使用する方法がある。しかし、前者では精度の問題、後者では採取試料の乱れの影響により、原位置の液状化強度比が適切に評価されないケースも多くある。本研究では一連の室内試験と既往研究の分析により、「同じ密度で異なる構造を有するセメンテーション効果の弱い砂質土供試体」の液状化強度比RLとせん断波速度Vsのそれぞれの比の間には、高い相関があることを確認した。本論文は、この関係を利用した原地盤の液状化強度比の推定に関するアイデアを示す。